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» Online Classroom   » Radar   » Public Discussion of Marine Radar   » Figure 11-32, annotated to show new SRM

Author Topic: Figure 11-32, annotated to show new SRM
David Burch

 - posted November 20, 2005 04:44 PM      Profile for David Burch           Edit/Delete Post 
It will help to refer to page 194 in the book and skim that section to follow these notes.

We have added the heading line and CPA values to the top part of the figure to show a better perspective on where these ERBL plots would be made relative to the center of the screen, and in both top and bottom we have added further annotations to show new values of SRM after the maneuver.


Note that this top picture is solving the problem right on the radar screen using the ERBL. Step by step instructions are in the caption to the figure in the text book.

We see a target approaching and by projecting its wake we conclude the CPA is too small (green one). We decide we are going to maneuver at point A to open the CPA (from the green value to the orange value). Then follow instructions in the book to figure how much to turn to the right as shown.

The book does not go beyond that, but we could also measure from this diagram the value of the expected new SRM after we turn -- without having to wait till the new wake develops. You might, for example, want that to predict the time of the new CPA without having to wait till the new wake develops.

That value is shown as the aqua line in the picture. You can reason through that this must be right since the true course and speed of the target does not change (that has nothing to do without maneuvers) and the length of our buoy trail will not change when we turn since we are not changing speed -- that is, after all, how we constructed the diagram. Thus we have the relative motion diagram for the new conditions all laid out for us already. We expect the wake of the target after we maneuver to look like the aqua line segment.


Now that we have done our extra work, the reader gets the opportunity to do some. If you are taking our online radar course, you will get these questions to think about.

Assume the wake trail is set for 6 minutes and it is nighttime. (You can print this page to help evaluate the picture or read it from the screen.)

(Q1) If we did not manuever as computed here but carried on same course and speed, how many minutes would it be till this vessel crossed our bow?

(Q2) At the time of this picture, what lights would we see for this vessel?

(Q3) What lights would we see when the vessel crosses our bow, assuming no maneuver?

(Q4) If we maneuver as planned, how many minutes till the vessel crossed our bow?

(Q5) If our speed is 10 kts and our course is 220 T, what is the true course and speed of the target vessel?

-- still same speed and wake interval, but now in the fog

(Q6) If it was pea soup fog and we could not see the vessel or its lights, did we turn in the correct direction?

(Q7) Regardless of the answer to (Q6), did we turn early enough?

(Q8) According to the Cockcroft diagram (p.209 in Radar for Mariners), what is the approximate turn we should have made?

(Q9) In the fog as we approached this vessel what maneuver if any should we expect of the other vessel as we approach each other.

(Q10) In clear weather, what manuever if any would we expect of the other vessel in this situation?



Note that the top and bottom examples are not for the same encounter, just a similar one. We can tell this since the true course and speed of the target is not the same in both cases.

This picture shows how to solve the same problem as above using an electronic chart system (ECS), sometimes called charting- or navigation-software. Step by step instructions are in the caption to the figure in the text book.


From: Starpath, Seattle, WA
David Burch

 - posted November 21, 2005 08:18 AM      Profile for David Burch           Edit/Delete Post 
Another aspect of this that we have not presented in the book, was how to decide what the new DRM should be to accomplish the desired CPA. This is in the Radar Trainer tutorial, but not the book.

The process is illustrated in the figure below. The top part is all you have to go by when you decide that you need to open the CPA, and the bottom part shows one way to figure out how to do it. This can be done by VRM and ERBL or on an echart or by traditional plotting. The bearing from the present target position to G or H would be the new DRMs to make good the new CPA. Note one is a turn to the right, the other a turn to the left...BUT we are not implying that you have the option of turning either way in this case or any other. Needless to say, most circumstances will specifically call for one and not the other. This just shows how you draw the lines.


You can also open the CPA in this case by slowing down. All targets turn upscreen when you slow down, which would open the CPA in this case.

From: Starpath, Seattle, WA

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